How do I disable texture filtering?
For the 3D view go to:


User preferences > System.
Then under the OpenGL sub header, you will find "MipMaps" make sure that box is unchecked.


For Blender-Internal go to:


Texture Buttons.
Image Sampling Panel.
Toggle MIP Map button.


Update

As of blender 2.71 texture nodes now have an option to change the interpolation method:



Using Closest will keep the pixels sharp.



Old Answer

For cycles, this is a needed feature. Here's a workaround (32 is the size of the texture) :